Recent developments in peptide chemistry have increased interest in the potential biological significance of these substances. Attribution of a peptide structure to many important hormones, releasing factors, chalones, kinins, specific behavior-inducing substances, and the possibility of the role of peptides in cancer therapy, suggest that these substances may play an important role in the regulation of biological functions. It has already been proven that some of the biologically active peptides are present in urine, which may be a convenient source for their isolation. The general aim of this study is the isolation, purification, identification and determination of the biological, therapeutic, and diagnostic significance of urinary peptides, especially their effect on normal and neoplastic cell growth. The peptides are isolated by ethanol extraction, gel filtration, free-flow high-voltage electrophoresis and thin-layer chromatography. Since only a small fraction of the urinary peptides is expected to have functional significance, a battery of bioassays will be set up to evaluate the activity on various normal and neoplastic cells (DNA, RNA and protein synthesis), animal tumors, intestinal and vascular smooth muscle, cardiac functions and behavior. The peptides proved active will be identified by amino acid analysis and sequence determination, and will be reproduced by synthesis. According to preliminary experiments, 10% of the 119 peptides studied up to now inhibit DNA, RNA, or protein synthesis in cultures of human osteosarcoma cells. Four substances of this group showed no inhibitory action on DNA synthesis in normal cells (human embryonal fibroblasts) and no effect on smooth muscle and heart. Six fractions produced remarkable inhibition of DNA synthesis (from 75% to 97%) in human leukemic cells. The active fractions have been selected for further study.